Mutant residues suppressing rho(0)-lethality in Kluyveromyces lactis occur at contact sites between subunits of F(1)-ATPase

Oxidative Phosphorylation Is Required for Powering Motility and Development of the Sleeping Sickness Parasite Trypanosoma brucei in the Tsetse Fly Vector

The single-celled parasite Trypanosoma brucei is transmitted by hematophagous tsetse flies. Life cycle progression from mammalian bloodstream form to tsetse midgut form and, subsequently, infective salivary gland form depends on complex developmental steps and migration within different fly tissues. As the parasite colonizes the glucose-poor insect midgut, ATP production is thought to depend on activation of mitochondrial amino acid catabolism via oxidative phosphorylation (OXPHOS). This process involves respiratory chain complexes and F₁F_o-ATP synthase and requires protein subunits of these complexes that are encoded in the parasite's mitochondrial DNA (kDNA). Here, we show that progressive loss of kDNA-encoded functions correlates with a decreasing ability to initiate and complete development in the tsetse. First, parasites with a mutated F₁F_o-ATP synthase with reduced capacity for OXPHOS can initiate differentiation from bloodstream to insect form, but they are unable to proliferate in vitro. Unexpectedly, these cells can still colonize the tsetse midgut. However, these parasites exhibit a motility defect and are severely impaired in colonizing or migrating to subsequent tsetse tissues. Second, parasites with a fully disrupted F₁F_o-ATP synthase complex that is completely unable to produce ATP by OXPHOS can still differentiate to the first insect stage in vitro but die within a few days and cannot establish a midgut infection in vivo. Third, parasites lacking kDNA entirely can initiate differentiation but die soon after. Together, these scenarios suggest that efficient ATP production via OXPHOS is not essential for initial colonization of the tsetse vector but is required to power trypanosome migration within the fly. IMPORTANCE African trypanosomes cause disease in humans and their livestock and are transmitted by tsetse flies. The insect ingests these parasites with its blood meal, but to be transmitted to another mammal, the trypanosome must undergo complex development within the tsetse fly and migrate from the insect's gut to its salivary glands. Crucially, the parasite must switch from a sugar-based diet while in the mammal to a diet based primarily on amino acids when it develops in the insect. Here, we show that efficient energy production by an organelle called the mitochondrion is critical for the trypanosome's ability to swim and to migrate through the tsetse fly. Surprisingly, trypanosomes with impaired mitochondrial energy production are only mildly compromised in their ability to colonize the tsetse fly midgut. Our study adds a new perspective to the emerging view that infection of tsetse flies by trypanosomes is more complex than previously thought.

The metabolic growth limitations of petite cells lacking the mitochondrial genome

Eukaryotic cells can survive the loss of their mitochondrial genome, but consequently suffer from severe growth defects. 'Petite yeasts', characterized by mitochondrial genome loss, are instrumental for studying mitochondrial function and physiology. However, the molecular cause of their reduced growth rate remains an open question. Here we show that petite cells suffer from an insufficient capacity to synthesize glutamate, glutamine, leucine and arginine, negatively impacting their growth. Using a combination of molecular genetics and omics approaches, we demonstrate the evolution of fast growth overcomes these amino acid deficiencies, by alleviating a perturbation in mitochondrial iron metabolism and by restoring a defect in the mitochondrial tricarboxylic acid cycle, caused by aconitase inhibition. Our results hence explain the slow growth of mitochondrial genome-deficient cells with a partial auxotrophy in four amino acids that results from distorted iron metabolism and an inhibited tricarboxylic acid cycle.

Systematic analysis reveals the prevalence and principles of bypassable gene essentiality

Gene essentiality is a variable phenotypic trait, but to what extent and how essential genes can become dispensable for viability remain unclear. Here, we investigate 'bypass of essentiality (BOE)' - an underexplored type of digenic genetic interaction that renders essential genes dispensable. Through analyzing essential genes on one of the six chromosome arms of the fission yeast Schizosaccharomyces pombe, we find that, remarkably, as many as 27% of them can be converted to non-essential genes by BOE interactions. Using this dataset we identify three principles of essentiality bypass: bypassable essential genes tend to have lower importance, tend to exhibit differential essentiality between species, and tend to act with other bypassable genes. In addition, we delineate mechanisms underlying bypassable essentiality, including the previously unappreciated mechanism of dormant redundancy between paralogs. The new insights gained on bypassable essentiality deepen our understanding of genotype-phenotype relationships and will facilitate drug development related to essential genes.

mPOS is a novel mitochondrial trigger of cell death - implications for neurodegeneration

In addition to its central role in energy metabolism, the mitochondrion has many other functions essential for cell survival. When stressed, the multifunctional mitochondria are expected to engender multifaceted cell stress with complex physiological consequences. Potential extra-mitochondrial proteostatic burdens imposed by inefficient protein import have been largely overlooked. Accumulating evidence suggests that a diverse range of pathogenic mitochondrial stressors, which do not directly target the core protein import machinery, can reduce cell fitness by disrupting the proteostatic network in the cytosol. The resulting stress, named mitochondrial precursor overaccumulation stress (mPOS), is characterized by the toxic accumulation of unimported mitochondrial proteins in the cytosol. Here, we review our current understanding of how mitochondrial dysfunction can impact the cytosolic proteome and proteostatic signaling. We also discuss the intriguing possibility that the mPOS model may help untangle the cause-effect relationship between mitochondrial dysfunction and cytosolic protein aggregation, which are probably the two most prominent molecular hallmarks of neurodegenerative disease.

Understanding structure, function, and mutations in the mitochondrial ATP synthase

The mitochondrial ATP synthase is a multimeric enzyme complex with an overall molecular weight of about 600,000 Da. The ATP synthase is a molecular motor composed of two separable parts: F₁ and F_o. The F₁ portion contains the catalytic sites for ATP synthesis and protrudes into the mitochondrial matrix. F_o forms a proton turbine that is embedded in the inner membrane and connected to the rotor of F₁. The flux of protons flowing down a potential gradient powers the rotation of the rotor driving the synthesis of ATP. Thus, the flow of protons though F_o is coupled to the synthesis of ATP. This review will discuss the structure/function relationship in the ATP synthase as determined by biochemical, crystallographic, and genetic studies. An emphasis will be placed on linking the structure/function relationship with understanding how disease causing mutations or putative single nucleotide polymorphisms (SNPs) in genes encoding the subunits of the ATP synthase, will affect the function of the enzyme and the health of the individual. The review will start by summarizing the current understanding of the subunit composition of the enzyme and the role of the subunits followed by a discussion on known mutations and their effect on the activity of the ATP synthase. The review will conclude with a summary of mutations in genes encoding subunits of the ATP synthase that are known to be responsible for human disease, and a brief discussion on SNPs.

Screen for mitochondrial DNA copy number maintenance genes reveals essential role for ATP synthase

The machinery of mitochondrial DNA (mtDNA) maintenance is only partially characterized and is of wide interest due to its involvement in disease. To identify novel components of this machinery, plus other cellular pathways required for mtDNA viability, we implemented a genome-wide RNAi screen in Drosophila S2 cells, assaying for loss of fluorescence of mtDNA nucleoids stained with the DNA-intercalating agent PicoGreen. In addition to previously characterized components of the mtDNA replication and transcription machineries, positives included many proteins of the cytosolic proteasome and ribosome (but not the mitoribosome), three proteins involved in vesicle transport, some other factors involved in mitochondrial biogenesis or nuclear gene expression, > 30 mainly uncharacterized proteins and most subunits of ATP synthase (but no other OXPHOS complex). ATP synthase knockdown precipitated a burst of mitochondrial ROS production, followed by copy number depletion involving increased mitochondrial turnover, not dependent on the canonical autophagy machinery. Our findings will inform future studies of the apparatus and regulation of mtDNA maintenance, and the role of mitochondrial bioenergetics and signaling in modulating mtDNA copy number.

Mutations on the N-terminal edge of the DELSEED loop in either the α or β subunit of the mitochondrial F1-ATPase enhance ATP hydrolysis in the absence of the central γ rotor

F(1)-ATPase is a rotary molecular machine with a subunit stoichiometry of α(3)β(3)γ(1)δ(1)ε(1). It has a robust ATP-hydrolyzing activity due to effective cooperativity between the three catalytic sites. It is believed that the central γ rotor dictates the sequential conformational changes to the catalytic sites in the α(3)β(3) core to achieve cooperativity. However, recent studies of the thermophilic Bacillus PS3 F(1)-ATPase have suggested that the α(3)β(3) core can intrinsically undergo unidirectional cooperative catalysis (T. Uchihashi et al., Science 333:755-758, 2011). The mechanism of this γ-independent ATP-hydrolyzing mode is unclear. Here, a unique genetic screen allowed us to identify specific mutations in the α and β subunits that stimulate ATP hydrolysis by the mitochondrial F(1)-ATPase in the absence of γ. We found that the F446I mutation in the α subunit and G419D mutation in the β subunit suppress cell death by the loss of mitochondrial DNA (ρ(o)) in a Kluyveromyces lactis mutant lacking γ. In organello ATPase assays showed that the mutant but not the wild-type γ-less F(1) complexes retained 21.7 to 44.6% of the native F(1)-ATPase activity. The γ-less F(1) subcomplex was assembled but was structurally and functionally labile in vitro. Phe446 in the α subunit and Gly419 in the β subunit are located on the N-terminal edge of the DELSEED loops in both subunits. Mutations in these two sites likely enhance the transmission of catalytically required conformational changes to an adjacent α or β subunit, thereby allowing robust ATP hydrolysis and cell survival under ρ(o) conditions. This work may help our understanding of the structural elements required for ATP hydrolysis by the α(3)β(3) subcomplex.

Absence of anionic phospholipids in Kluyveromyces lactis cells is fatal without F1-catalysed ATP hydrolysis

We have shown in previous research that the loss of phosphatidylglycerol and cardiolipin caused by disruption of the PGS1 gene is lethal for the petite-negative yeast Kluyveromyces lactis . This present study demonstrates the role and mechanism of atp2.1 in the suppression of pgs1 lethality in K. lactis cells. Phenotypic characterization has shown that a strain lacking the phosphatidylglycerolphosphate synthase (atp2.1pgs1Δ) possessed a markedly impaired respiratory chain, very low endogenous respiration, and uncoupled mitochondria. As a result the mutant strain was unable to generate a sufficient mitochondrial membrane potential via respiration. The atp2.1 suppressor mutation enabled an increase in the affinity of F(1)-ATPase for ATP in the hydrolytic reaction, resulting in the maintenance of sufficient membrane potential for the biogenesis of mitochondria and survival of cells lacking anionic phospholipid biosynthesis.

Miltefosine induces apoptosis-like cell death in yeast via Cox9p in cytochrome c oxidase

Miltefosine has antifungal properties and potential for development as a therapeutic for invasive fungal infections. However, its mode of action in fungi is poorly understood. We demonstrate that miltefosine is rapidly incorporated into yeast, where it penetrates the mitochondrial inner membrane, disrupting mitochondrial membrane potential and leading to an apoptosis-like cell death. COX9, which encodes subunit VIIa of the cytochrome c oxidase (COX) complex in the electron transport chain of the mitochondrial membrane, was identified as a potential target of miltefosine from a genomic library screen of the model yeast Saccharomyces cerevisiae. When overexpressed in S. cerevisiae, COX9, but not COX7 or COX8, led to a miltefosine-resistant phenotype. The effect of miltefosine on COX activity was assessed in cells expressing different levels of COX9. Miltefosine inhibited COX activity in a dose-dependent manner in Cox9p-positive cells. This inhibition most likely contributed to the miltefosine-induced apoptosis-like cell death.

A functionally important hydrogen-bonding network at the betaDP/alphaDP interface of ATP synthase

ATP synthase uses a unique rotary mechanism to couple ATP synthesis and hydrolysis to transmembrane proton translocation. The F1 subcomplex has three catalytic nucleotide binding sites, one on each beta subunit, at the interface to the adjacent alpha subunit. In the x-ray structure of F1 (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628), the three catalytic beta/alpha interfaces differ in the extent of inter-subunit interactions between the C termini of the beta and alpha subunits. At the closed betaDP/alphaDP interface, a hydrogen-bonding network is formed between both subunits, which is absent at the more open betaTP/alphaTP interface and at the wide open betaE/alphaE interface. The hydrogen-bonding network reaches from betaL328 (Escherichia coli numbering) and betaQ441 via alphaQ399, betaR398, and alphaE402 to betaR394, and ends in a cation/pi interaction between betaR394 and alphaF406. Using mutational analysis in E. coli ATP synthase, the functional importance of the betaDP/alphaDP hydrogen-bonding network is demonstrated. Its elimination results in a severely impaired enzyme but has no pronounced effect on the binding affinities of the catalytic sites. A possible role for the hydrogen-bonding network in coupling of ATP synthesis/hydrolysis and rotation will be discussed.

Mechanism of inhibition of bovine F1-ATPase by resveratrol and related polyphenols

The structures of F(1)-ATPase from bovine heart mitochondria inhibited with the dietary phytopolyphenol, resveratrol, and with the related polyphenols quercetin and piceatannol have been determined at 2.3-, 2.4- and 2.7-A resolution, respectively. The inhibitors bind to a common site in the inside surface of an annulus made from loops in the three alpha- and three beta-subunits beneath the "crown" of beta-strands in their N-terminal domains. This region of F(1)-ATPase forms a bearing to allow the rotation of the tip of the gamma-subunit inside the annulus during catalysis. The binding site is a hydrophobic pocket between the C-terminal tip of the gamma-subunit and the beta(TP) subunit, and the inhibitors are bound via H-bonds mostly to their hydroxyl moieties mediated by bound water molecules and by hydrophobic interactions. There are no equivalent sites between the gamma-subunit and either the beta(DP) or the beta(E) subunit. The inhibitors probably prevent both the synthetic and hydrolytic activities of the enzyme by blocking both senses of rotation of the gamma-subunit. The beneficial effects of dietary resveratrol may derive in part by preventing mitochondrial ATP synthesis in tumor cells, thereby inducing apoptosis.

Mutations in the Atp1p and Atp3p subunits of yeast ATP synthase differentially affect respiration and fermentation in Saccharomyces cerevisiae

ATP1-111, a suppressor of the slow-growth phenotype of yme1Delta lacking mitochondrial DNA is due to the substitution of phenylalanine for valine at position 111 of the alpha-subunit of mitochondrial ATP synthase (Atp1p in yeast). The suppressing activity of ATP1-111 requires intact beta (Atp2p) and gamma (Atp3p) subunits of mitochondrial ATP synthase, but not the stator stalk subunits b (Atp4p) and OSCP (Atp5p). ATP1-111 and other similarly suppressing mutations in ATP1 and ATP3 increase the growth rate of wild-type strains lacking mitochondrial DNA. These suppressing mutations decrease the growth rate of yeast containing an intact mitochondrial chromosome on media requiring oxidative phosphorylation, but not when grown on fermentable media. Measurement of chronological aging of yeast in culture reveals that ATP1 and ATP3 suppressor alleles in strains that contain mitochondrial DNA are longer lived than the isogenic wild-type strain. In contrast, the chronological life span of yeast cells lacking mitochondrial DNA and containing these mutations is shorter than that of the isogenic wild-type strain. Spore viability of strains bearing ATP1-111 is reduced compared to wild type, although ATP1-111 enhances the survival of spores that lacked mitochondrial DNA.

A functional core of the mitochondrial genome maintenance protein Mgm101p in Saccharomyces cerevisiae determined with a temperature-conditional allele

Analysis of Mgm101p isolated from mitochondria shows that the mature protein of 27.6 kDa lacks 22 amino acids from the N-terminus. This mitochondrial targeting sequence has been incorporated in the design of oligonucleotides used to determine a functional core of Mgm101p. Progressive deletions, although retaining the targeting sequence, reveal that 76 N-terminal and six C-terminal amino acids of Mgm101p can be removed without altering the ability to complement an mgm101-1(ts) temperature-sensitive mutant. However, this active core is unable to complement mgm101 null mutants, suggesting that the Mgm101p might need to form a dimer or multimer to be functional in vivo. The active core, enriched in basic residues, contains 165 amino acids with a pI of 9.2. Alignment with 22 Mgm101p sequences from other lower eukaryotes shows that a number of amino acids are highly conserved in this region. Random mutagenesis confirms that certain critical amino acids required for function are invariant across the 23 proteins. Searches in the PFAM database revealed a low level of structural similarity between the active core and the Rad52 protein family.

Mitochondrial genome integrity mutations uncouple the yeast Saccharomyces cerevisiae ATP synthase

The mitochondrial ATP synthase is a molecular motor, which couples the flow of protons with phosphorylation of ADP. Rotation of the central stalk within the core of ATP synthase effects conformational changes in the active sites driving the synthesis of ATP. Mitochondrial genome integrity (mgi) mutations have been previously identified in the alpha-, beta-, and gamma-subunits of ATP synthase in yeast Kluyveromyces lactis and trypanosome Trypanosoma brucei. These mutations reverse the lethality of the loss of mitochondrial DNA in petite negative strains. Introduction of the homologous mutations in Saccharomyces cerevisiae results in yeast strains that lose mitochondrial DNA at a high rate and accompanied decreases in the coupling of the ATP synthase. The structure of yeast F1-ATPase reveals that the mgi residues cluster around the gamma-subunit and selectively around the collar region of F1. These results indicate that residues within the mgi complementation group are necessary for efficient coupling of ATP synthase, possibly acting as a support to fix the axis of rotation of the central stalk.

Formation of an energized inner membrane in mitochondria with a gamma-deficient F1-ATPase

Eukaryotic cells require mitochondrial compartments for viability. However, the budding yeast Saccharomyces cerevisiae is able to survive when mitochondrial DNA suffers substantial deletions or is completely absent, so long as a sufficient mitochondrial inner membrane potential is generated. In the absence of functional mitochondrial DNA, and consequently a functional electron transport chain and F(1)F(o)-ATPase, the essential electrical potential is maintained by the electrogenic exchange of ATP(4-) for ADP(3-) through the adenine nucleotide translocator. An essential aspect of this electrogenic process is the conversion of ATP(4-) to ADP(3-) in the mitochondrial matrix, and the nuclear-encoded subunits of F(1)-ATPase are hypothesized to be required for this process in vivo. Deletion of ATP3, the structural gene for the gamma subunit of the F(1)-ATPase, causes yeast to quantitatively lose mitochondrial DNA and grow extremely slowly, presumably by interfering with the generation of an energized inner membrane. A spontaneous suppressor of this slow-growth phenotype was found to convert a conserved glycine to serine in the beta subunit of F(1)-ATPase (atp2-227). This mutation allowed substantial ATP hydrolysis by the F(1)-ATPase even in the absence of the gamma subunit, enabling yeast to generate a twofold greater inner membrane potential in response to ATP compared to mitochondria isolated from yeast lacking the gamma subunit and containing wild-type beta subunits. Analysis of the suppressing mutation by blue native polyacrylamide gel electrophoresis also revealed that the alpha(3)beta(3) heterohexamer can form in the absence of the gamma subunit.

The F1-ATP synthase complex in bloodstream stage trypanosomes has an unusual and essential function

Survival of bloodstream form Trypanosoma brucei, the agent of African sleeping sickness, normally requires mitochondrial gene expression, despite the absence of oxidative phosphorylation in this stage of the parasite's life cycle. Here we report that silencing expression of the alpha subunit of the mitochondrial F(1)-ATP synthase complex is lethal for bloodstream stage T. brucei as well as for T. evansi, a closely related species that lacks mitochondrial protein coding genes (i.e. is dyskinetoplastic). Our results suggest that the lethal effect is due to collapse of the mitochondrial membrane potential, which is required for mitochondrial function and biogenesis. We also identified a mutation in the gamma subunit of F(1) that is likely to be involved in circumventing the requirement for mitochondrial gene expression in another dyskinetoplastic form. Our data reveal that the mitochondrial ATP synthase complex functions in the bloodstream stage opposite to that in the insect stage and in most other eukaryotes, namely using ATP hydrolysis to generate the mitochondrial membrane potential.

Kinetic properties of F1-ATPase influence the ability of yeasts to grow in anoxia or absence of mtDNA

A mechanism for hypoxia survival by eukaryotic cells is suggested from studies on the petite mutation of yeasts. Previous work has shown that mutations in the alpha, beta and gamma subunit genes of F1-ATPase can suppress lethality due to loss of the mitochondrial genome from the petite-negative yeast Kluyveromyceslactis. Here it is reported that suppressor mutations appear to increase the affinity of F1-ATPase for ATP. Extension of this study to other yeasts shows that petite-positive species have a higher affinity for ATP in the hydrolysis reaction than petite-negative species. Possession of a F1-ATPase with a low K(m) for ATP is considered to be an adaptation for hypoxic growth, enabling maintenance of the mitochondrial inner membrane potential, deltapsi, by enhanced export of protons through F1F0-ATP synthase connected to increased ATP hydrolysis at low substrate concentration.

Genetic conservation versus variability in mitochondria: the architecture of the mitochondrial genome in the petite-negative yeast Schizosaccharomyces pombe

The great amount of molecular information and the many molecular genetic techniques available make Schizosaccharomyces pombe an ideal model eukaryote, complementary to the budding yeast Saccharomyces cerevisiae. In particular, mechanisms involved in mitochiondrial (mt) biogenesis in fission yeast are more similar to higher eukaryotes than to budding yeast. In this review, recent findings on mt morphogenesis, DNA replication and gene expression in this model organism are summarised. A second aspect is the organisation of the mt genome in fission yeast. On the one hand, fission yeast has a strong tendency to maintain mtDNA intact; and, on the other hand, the mt genomes of naturally occurring strains show a great variability. Therefore, the molecular mechanisms behind the susceptibility to mutations in the mtDNA and the mechanisms that promote sequence variations during the evolution of the genome in fission yeast mitochondria are discussed.

F1-catalysed ATP hydrolysis is required for mitochondrial biogenesis in Saccharomyces cerevisiae growing under conditions where it cannot respire

Mutant strains of yeast Saccharomyces cerevisiae lacking a functional F1-ATPase were found to grow very poorly under anaerobic conditions. A single amino acid replacement (K222 > E222) that locally disrupts the adenine nucleotide catalytic site in the beta-F1 subunit was sufficient to compromise anaerobic growth. This mutation also affected growth in aerated conditions when ethidium bromide (an intercalating agent impairing mtDNA propagation) or antimycin (an inhibitor of respiration) was included in the medium. F1-deficient cells forced to grow in oxygen-limited conditions were shown to lose their mtDNA completely and to accumulate Hsp60p mainly under its precursor form. Fluorescence microscopy analyses with a modified GFP containing a mitochondrial targeting presequence revealed that aerobically growing F1-deficient cells stopped importing the GFP when antimycin was added to the medium. Finally, after total inactivation of the catalytic alpha3beta3 subcomplex of F1, mitochondria could no longer be energized by externally added ATP because of either a block in assembly or local disruption of the adenine nucleotide processing site. Altogether these data strengthen the notion that in the absence of respiration, and whether the proton translocating domain (F0) of complex V is present or not, F1-catalysed hydrolysis of ATP is essential for the occurrence of vital cellular processes depending on the maintenance of an electrochemical potential across the mitochondrial inner membrane.

Natural and induced dyskinetoplastic trypanosomatids: how to live without mitochondrial DNA

Salivarian trypanosomes are the causative agents of several diseases of major social and economic impact. The most infamous parasites of this group are the African subspecies of the Trypanosoma brucei group, which cause sleeping sickness in humans and nagana in cattle. In terms of geographical distribution, however, Trypanosoma equiperdum and Trypanosoma evansi have been far more successful, causing disease in livestock in Africa, Asia, and South America. In these latter forms the mitochondrial DNA network, the kinetoplast, is altered or even completely lost. These natural dyskinetoplastic forms can be mimicked in bloodstream form T. brucei by inducing the loss of kinetoplast DNA (kDNA) with intercalating dyes. Dyskinetoplastic T. brucei are incapable of completing their usual developmental cycle in the insect vector, due to their inability to perform oxidative phosphorylation. Nevertheless, they are usually as virulent for their mammalian hosts as parasites with intact kDNA, thus questioning the therapeutic value of attempts to target mitochondrial gene expression with specific drugs. Recent experiments, however, have challenged this view. This review summarises the data available on dyskinetoplasty in trypanosomes and revisits the roles the mitochondrion and its genome play during the life cycle of T. brucei.

Dual mutations reveal interactions between components of oxidative phosphorylation in Kluyveromyces lactis

Loss of mtDNA or mitochondrial protein synthesis cannot be tolerated by wild-type Kluyveromyces lactis. The mitochondrial function responsible for rho(0)-lethality has been identified by disruption of nuclear genes encoding electron transport and F(0)-ATP synthase components of oxidative phosphorylation. Sporulation of diploid strains heterozygous for disruptions in genes for the two components of oxidative phosphorylation results in the formation of nonviable spores inferred to contain both disruptions. Lethality of spores is thought to result from absence of a transmembrane potential, Delta Psi, across the mitochondrial inner membrane due to lack of proton pumping by the electron transport chain or reversal of F(1)F(0)-ATP synthase. Synergistic lethality, caused by disruption of nuclear genes, or rho(0)-lethality can be suppressed by the atp2.1 mutation in the beta-subunit of F(1)-ATPase. Suppression is viewed as occurring by an increased hydrolysis of ATP by mutant F(1), allowing sufficient electrogenic exchange by the translocase of ADP in the matrix for ATP in the cytosol to maintain Delta Psi. In addition, lethality of haploid strains with a disruption of AAC encoding the ADP/ATP translocase can be suppressed by atp2.1. In this case suppression is considered to occur by mutant F(1) acting in the forward direction to partially uncouple ATP production, thereby stimulating respiration and relieving detrimental hyperpolarization of the inner membrane. Participation of the ADP/ATP translocase in suppression of rho(0)-lethality is supported by the observation that disruption of AAC abolishes suppressor activity of atp2.1.